Arun

The Arun is a major trans-Himalayan river that rises on the Tibetan Plateau, where it is known as the Phung Chu or Bum-chu (also transliterated Peng Qu). It originates in the high country of southern Tibet near Nyalam County and flows generally eastward and then southward across the arid Tibetan tableland before turning toward the main Himalayan chain.

The river enters Nepal at an elevation of roughly 3,500 metres (about 11,500 ft). South of its crossing it carves directly through the principal Himalayan range, threading the gap between two of the world's great mountain massifs, Makalu to the west and Kangchenjunga to the east. The valley it has cut here is exceptionally deep, with several thousand metres of vertical relief between the river bed and the surrounding peaks, and the Arun and adjoining Barun valleys are frequently cited among the world's deepest.

After breaching the Himalaya the Arun descends through eastern Nepal and is joined from the west by the Sun Koshi and from the east by the Tamur. The three rivers meet near Tribeni, in the area of Barahakshetra and the Chatra gorge, where their combined waters form the Sapta Koshi (the "seven Koshi"), the largest river system contributing to the Ganges from Nepal. From there the unified river flows out onto the Gangetic plain and into India.

The Arun drains a very large catchment that straddles the Nepal-China (Tibet) border, with the great majority of its basin lying north of the main Himalayan crest. More than 80 percent of the drainage area sits in the rain shadow of the Himalaya, where average annual rainfall on the Tibetan side is only on the order of 300 millimetres. Because so much of the basin is high, cold and glaciated, the Arun has the greatest snow- and ice-covered area of any river basin in Nepal, and meltwater makes an important contribution to its flow.

Within the Sapta Koshi system, the Arun supplies roughly 37 percent of the combined discharge, compared with about 44 percent from the Sun Koshi and about 19 percent from the Tamur. Its flow rises sharply once it leaves the dry Tibetan plateau and enters the monsoon-fed mountains and middle hills of eastern Nepal, where summer rains greatly augment the snow- and glacier-melt it carries down from Tibet.

On the Nepalese side the most significant tributary is the Barun River, which joins the Arun from the west and drains the glaciers and high basins below Mount Makalu. Together with the Tamur, Sun Koshi and the latter's own northern tributaries (such as the Dudh Koshi, Tama Koshi, Bhote Koshi, Likhu and Indrawati), the Arun forms one of the three principal arms of the Koshi network upstream of the Chatra gorge.

The steep gradient and large, reliable flow of the Arun make it one of Nepal's most important rivers for hydroelectric development. Its deep gorges and high head over short distances are well suited to run-of-river and storage schemes, and the corridor has long been identified as a focus of Nepal's hydropower ambitions.

The flagship project on the river is Arun-3 (Arun III), a 900 MW scheme under construction in Sankhuwasabha District in Koshi Province. It is being built by SJVN Arun-3 Power Development Company, a subsidiary of India's SJVN Ltd., on a build-own-operate-transfer (BOOT) basis under which the developer operates the plant for 30 years before handing it to the Nepalese government. The design centres on a concrete gravity dam about 70 metres high and 466 metres wide, with a small reservoir storing roughly 13.94 million cubic metres of water, feeding an underground powerhouse equipped with four vertical Francis turbines of 225 MW each.

Water is conveyed to the powerhouse through a headrace tunnel about 11.74 km long and 9.5 metres in diameter, followed by steel-lined pressure shafts and penstocks. The project is expected to generate on the order of 4,019 million units (around 4,000 GWh) of electricity per year, with about 21.9 percent of that energy supplied to Nepal free of cost over the operating period and much of the remainder evacuated to India via a 400 kV transmission line running from the project area through Dhalkebar toward Muzaffarpur. An earlier version of an Arun-3 project in the 1990s, backed by the World Bank, was abandoned in the mid-1990s amid cost, governance and environmental concerns before the present scheme was revived under a 2014 development agreement.

As a headwater of the Sapta Koshi, the Arun belongs to one of the great sacred river systems feeding the Ganges, and the broader Koshi basin holds deep significance in the Hindu traditions of Nepal and northern India. The confluence region around Barahakshetra, near where the Arun, Sun Koshi and Tamur unite, is a long-established Hindu pilgrimage area associated with temple worship and ritual bathing at the meeting of the rivers.

The high country drained by the Arun and its Barun tributary is also revered in Tibetan Buddhist tradition. The Barun valley, in the Makalu region, is venerated as a beyul, a sacred "hidden valley." In the Nyingma school of Tibetan Buddhism, beyuls are remote sanctuaries believed to have been blessed by the 8th-century master Padmasambhava (Guru Rinpoche) as places of refuge and spiritual practice; the Barun/Khembalung area is one such consecrated landscape, drawing both Buddhist and Hindu pilgrims.

The Arun corridor lies within the homeland of the Kirat peoples of eastern Nepal, including Rai and Limbu communities, whose settlements, languages and traditions shape the cultural geography of the valley. The river thus connects a Tibetan Buddhist sacred upland with the Hindu and Kirat cultural world of the Nepalese hills and the holy waters of the Koshi-Ganges system downstream.

The Arun basin spans an extraordinary altitudinal and ecological range, from high, glaciated Tibetan and Himalayan terrain down through temperate forests to the warmer middle hills of eastern Nepal. Much of its upper Nepalese catchment falls within or beside Makalu Barun National Park, established in 1992 and centred on Mount Makalu, the world's fifth-highest peak. The park protects habitats ranging from subtropical lowlands to alpine zones and shelters threatened species such as the snow leopard and red panda, along with several hundred recorded bird species, making the Arun-Barun region one of the most biodiverse parts of the Himalaya.

The river's antecedent character, having maintained its course while the Himalaya rose around it, gives it exceptionally steep gradients and deep gorges. These same features create significant hazards: the catchment is prone to landslides, debris flows and flash flooding, and the high glaciated headwaters carry the risk of glacial lake outburst floods (GLOFs) that can surge down the narrow valleys with little warning. The wider Koshi system into which the Arun drains is notorious downstream for shifting its course and for severe monsoon flooding on the plains.

Large infrastructure on the river, including the Arun-3 scheme, raises environmental and social questions over altered flows, sediment transport, fish migration and the resilience of dams and tunnels in a seismically active, flood- and landslide-prone mountain setting. Climate change adds further uncertainty, as warming affects the snow and glacier melt that sustain much of the Arun's flow and may change the frequency of extreme events in the basin.